An organic light emitting diode (simply referred to as OLED) is an organic thin-film electroluminescent device, which has the advantages of simple production process, low cost, easy formation of flexible structure, wide view angle, etc.; and therefore, the display technology using organic light emitting diodes has become an important display technology.
As shown in FIG. 1, an OLED light emitting device comprises a anode 1, a cathode 2, and a light emitting layer 3 located between the anode 1 and the cathode 2. The principle for light emission is that holes (h) injected from the anode 1 and electrons (e) injected from the cathode 2 are combined at a recombination area of the light emitting layer 3 to form excitons, and the excitons decay to radiate photons so that this OLED light emitting device emits light.
However, investigators find that the mobility of holes in the light emitting layer 3 and the mobility of electrons in the light emitting layer 3 are not the same due to the influence of the material of the light emitting layer 3. On this basis, on the one hand, the difference in mobility will result in different numbers of holes and of electrons injected to the light emitting layer 3, and the injection imbalance of carriers (i.e., electrons and holes) will severely affect the performance and the lifetime of device.
On the other hand, the difference in mobility will result in the formation of excitons by recombination of part of holes and electrons at the interface between the light emitting layer 3 and the cathode 2 or at the interface between the light emitting layer 3 and the anode 1. For example, as shown in FIG. 2, in the case where the mobility of holes is greater than the mobility of electrons, excitons are formed by part of holes and electrons in a recombination area 31 at the interface between the light emitting layer 3 and the cathode 2 due to the relatively fast transport of holes, which are injected from the anode 1, in the light emitting layer 3. As shown in FIG. 3, in the case where the mobility of electrons is greater than the mobility of holes, excitons are formed by part of holes and electrons in a recombination area 31 at the interface between the light emitting layer 3 and the anode 1 due to the relatively fast transport of electrons, which are injected from the cathode 2, in the light emitting layer 3. The severe accumulation of excitons at the interface will cause the quench of organic light emitting diode, further affecting the performance and the lifetime of device.